1. Field of the Invention
The present invention relates to a thermistor, and more particularly, to a transistor heat controlling circuit for preventing a power transistor from heating up using a semiconductor thermistor, and an application circuit of the transistor heat controlling circuit.
2. Description of the Related Art
A thermistor is a temperature sensor that uses temperature dependency of a semiconductor, i.e., a representative semiconductor sensor device, which has been most widely used since semiconductors first appeared. The temperature dependency indicates that the resistance of the thermistor exponentially decreases with an increase in temperature. A commonly used thermistor with 2 terminals is formed of ceramic and solder, and a diode using a Si p-n junction is installed as a substitute for a thermistor in an integrated circuit (IC).
A thin-film-type thermistor is accurate from room temperature to at least 200° C. or more and is of micro-miniature size. A chip-type thermistor device may be manufactured using a temperature-sensitive package so as to be conveniently used.
Since a power transistor supplies power, the power transistor generates a large amount of heat due to over-current, which is preventable by a thermistor. However, if the thermistor is adhered to a base of the power transistor, the thermistor functions as a temperature sensor, and thus high current does not flow into the thermistor. If high current flows in the thermistor, the temperature of the thermistor rises and thus the thermistor is not accurate.
Also, if a transistor is adhered to the base of the power transistor, a p-n junction of the transistor does not greatly depend on temperature, and the resistance of the base of the power transistor is high. As a result, a power controlling effect of the power transistor is not high.
A controlling method using a thermistor operating with a temperature and a transistor operating with a voltage requires a comparator and a precise reference voltage. Therefore, an IC becomes complicated.